Paper and paperboard products (referred to hereinafter as "paper" for purposes of simplicity) made from cellulosic materials owe their strength, in part, to inter-fiber hydrogen bonds which form when water is drained from an aqueous slurry of fibers on a paper machine producing a web of fibers which is then pressed and is dried. Because the hydrogen bonds between fibers are relatively weak, the bonds are easily broken by the addition of water to paper which makes paper ideally suited for recycle.
Unfortunately, paper made using cellulosic fibers obtained from recycle or waste paper sources does not exhibit the same degree of strength as paper made from virgin pulp. Each time wood fibers are recycled, some of the bonding strength inherent in the fibers is lost. Loss in bonding strength may be due to structural damage to the fibers caused by repeated refining or other mechanical stress and/or due to a change in crystal structure of the cellulose in the fibers as a result of treatment with various agents and/or heating the pulp or paper. The change in crystal structure of the fibers is evidenced by the decreased ability of recycled fibers to swell.
Cellulosic fibers made directly from wood sources may also exhibit relatively weak strength properties due to the paper manufacturing methods or properties of the pulp. In order to increase the bonding strength of cellulosic fibers, particularly recycled cellulosic fibers, dry strength agents such as cationic starches have been added to the fibers. However, there is a pronounced decrease in retention of many dry strength agents on the fibers during paper formation when more than about 1 wt.% of the agent is applied, which limits the level of improvement achievable using such additives.
Cellulosic fibers have been chemically modified by treatment with alphachloroacetic acid or succinic anhydride dissolved in a solvent in an attempt to increase the bonding of cationic dry strength agents to the fibers so that more is retained in the web forming on the machine. However, chemical modification by such techniques tends to reduce the dimensional stability of the fibers. Because of the loss of dimensional stability, the fibers exhibit greater swellability resulting in paper products which change their dimensions more dramatically with changes in moisture and/or temperature.
It is therefore an object of the invention to provide a method for upgrading the quality of paper containing secondary fibers.
Another object of the invention is to provide a method for increasing the strength of paper products.
Still another object of the invention is to increase the interchain hydrogen bonding characteristics of cellulosic fibers formed into a paper web from wood pulp without adversely affecting the swellability of the fibers.
Another object of the invention is to provide high strength paper products made from recycled cellulosic fibers.
Yet another object of the invention is to provide a method for producing high strength pulp for paper products which is substantially independent of the source of the pulp fibers.
Still another object of the invention is to provide a method for increasing the ability of cellulosic fibers to bond to dry strength agents or wet strength agents.